1. Field of the Invention
The present invention relates to TiAl based alloys, a production process therefor, and a rotor blade using the same.
2. Description of Related Art
Recently, as materials used for a turbine blade of a turbocharger, a turbine engine or the like, and materials used for future aircraft, TiAl based alloys, being lightweight (specific gravity of about 4) and having excellent heat-resistance, are attracting much attention. In particular, in the case of a large blade, as the constituent material of the blade become lighter, the centrifugal stress becomes smaller, thus enabling improvement in the maximum attainable rpm, an increase in blade area, and a decrease in applied stress on the disk portion.
This TiAl based alloy is an alloy composed mainly of TiAl and Ti3Al, which is an intermetallic compound having excellent high temperature strength. As described above, it has excellent heat resistance, but has a problem in that ductility at room temperature is poor. Therefore, various measures have been heretofore taken, such as control of the microsstructure or ternary addition. For example, in Japanese Unexamined Patent Application, First Publication No. Hei 6-49565, there is disclosed a technique in which Cr or V is added as the ternary addition, in order to improve the ductility of the TiAl based alloy at normal temperature. Furthermore, a laminated structure (lamellar structure) region obtained by alternately laminating the TiAl phase and the Ti3Al phase is formed in a matrix structure in order to improve the strength. Moreover, Kim (Young-Won Kim. Intermetallics. (6) 1998 pp. 623-628) has reported that in a TiAl alloy having a lamellar grain with a mean grain diameter of from 30 to 3000 xcexcm, as the mean grain diameter of the lamellar grains increases, the ductility and tensile stress at a room temperature decrease.
The case of the above described technique however, is not sufficient in view of improvement in ductility at a normal temperature. In particular, with a blade used for an engine for industrial use or the like, foreign matter such as sludge may collide with the blade at the time of operation, or at the time of production of the blade, the blade may be broken due to impact at the time of fixing the blade to the outer periphery of the disk with a hammer. Hence, it becomes necessary to improve the impact property of the TiAl based alloy. With the above technique however, it is difficult to improve the impact property.
Furthermore, in many cases the TiAl based alloy has been heretofore produced by casting. However the casting structure is generally large, and there is a tendency for the impact property of a material to decrease further. In the case of casting, production of small parts such as vehicle parts is relatively easy. However production of large parts has been difficult due to problems with flowability of the molten metal in the mold. On the other hand, isothermal forging is also commonly used as a forging method of the TiAl based alloy. Here, in order to develop a lamellar structure, it is necessary to pass through a zone in which the xcex1-phase exists. With the isothermal forging, however, there is a problem in that since processing at a high temperature of 1150xc2x0 C. or higher is not possible due to problems of the apparatus, the lamellar structure necessary for improvement of the mechanical property is not developed in the forged material. In addition, production of large parts is also difficult.
The present inventors consider that it is essential to form the above described lamellar grains in the matrix, sin order to improve the strength of the TiAl based alloy. Based on this assumption, the present inventors have changed the mean grain diameter of the lamellar grains to various sizes, and have found that the ductility at room temperature, in particular, the impact property can be greatly improved for a predetermined mean grain diameter, thereby concluding the present invention.
Moreover, the present invention is in conceiving as a method for reducing the mean grain diameter of the lamellar structure, one wherein a TiAl based alloy material is held in an equilibrium temperature range of an xcex1 phase or in an equilibrium temperature range of an (xcex1+xcex2) phase, and then the material is subjected to high-speed plastic working in the cooling process thereafter. The invention is also in finding a specific method for this method.
That is to say, it is an object of the present invention to solve the above described problems in the TiAl based alloy and to provide a TiAl based alloy excellent in workability, and with excellent strength as well as an improvement in ductility at room temperature, in particular, an improvement in the impact properties at room temperature, and a production process therefor.
It is an another object of the present invention to provide a blade comprising a TiAl based alloy having improved impact properties.
To achieve the above objects, the TiAl based alloy of the present invention is characterized by having a fine structure in which lamellar grains having a mean grain diameter of from 1 to 50 xcexcm are closely arranged, with an xcex12 phase and a xcex3 phase being laminated therein alternately. More specifically, the TiAl based alloy of the present invention is characterized by two kinds of fine structures, one being a structure form (hereinafter, referred to as xe2x80x9cstructure 1xe2x80x9d) in which lamellar grains having a mean grain diameter of from 1 to 50 xcexcm are closely arranged, with the xcex12 phase and the xcex3 phase being laminated therein alternately, and the other being a structure form (hereinafter, referred to as xe2x80x9cstructure 2xe2x80x9d) in which a matrix composed mainly of a xcex2 phase fills the gaps between the lamellar grains in the form of net work, and the ratio of this matrix is not smaller than 10% and not larger than 40%.
By having such a microstructure, the strength is improved by means of the lamellar grains themselves formed in the metal structure, and since the lamellar grains having a small grain diameter distribute closely and finely, ductility at room temperature, in particular, impact resistance is improved. As other properties, with the structure 1, since the high temperature strength increases, it can be used as a turbine blade of a gas turbine or the like. Moreover, with the structure 2, high temperature deformability is improved due to the effect of the xcex2 phase between the lamellar grains, making plastic working easy. However, the creep strength slightly decreases. Therefore, it can be used as a turbine blade of a steam turbine or the like having a low upper limit for the operating temperature.
In order to achieve the above objects, one of the TiAl based alloys of the present invention may be a TiAl based alloy having a composition comprising 40 to 48 atomic % of Al, 5 to 10 atomic % of one or more kinds selected from Cr and V, with the remainder being Ti and inevitable impurities. The TiAl based alloy having this composition has an equilibrium range of xcex1 phase or (xcex1+xcex2) phase, which is wide at a high temperature. Moreover, according to a production method of the present invention described below, this TiAl based alloy is easily subjected to high-speed plastic working, and becomes a fine microstructure in which lamellar grains are closely arranged. As a result, a TiAl based alloy having excellent ductility at a room temperature, and in particular, excellent impact properties can be obtained. The structure 1 can be obtained by holding the TiAl based alloy in the xcex1 region, and the structure 2 can be obtained by holding the TiAl based alloy in the (xcex1+xcex2) region.
Another TiAl based alloy of the present invention may be a TiAl based alloy having a composition comprising 38 to 48 atomic % of Al, 4 to 10 atomic % of Mn, with the remainder being Ti and inevitable impurities. Also in the TiAl based alloy having this composition, the high-temperature equilibrium range of the xcex1 phase or (xcex1+xcex2) phase exists, and similarly, by holing the TiAl based alloy in the xcex1 region, the structure 1 can be obtained, and by holing the TiAl based alloy in the (xcex1+xcex2) region, the structure 2 can be obtained. With this structure, since the structure 1 and structure 2 are less hard than the above described TiAl based alloy, the machinability and impact resistance are improved. However, the high temperature strength slightly decreases. Therefore, the structure 1 and structure 2 are suitable for applications where this TiAl based alloy is used at a slightly lower temperature than the above described TiAl based alloy.
An other TiAl based alloy of the present invention is a TiAl based alloy relating to the above described two kinds of TiAl based alloys, further containing one or more kinds of elements selected from the group consisting of C, Si, Ni, W, Nb, B, Hf, Ta, and Zr in an amount of from 0.1 to 3 atomic % in total.
By adding these elements in a small amount, the high temperature strength, the creep strength and the oxidation resistance can be increased.
The TiAl based alloy of the present invention has a Charpy impact value specified in JIS-Z2242, of 3J or higher at a room temperature, and 5J or higher is also achievable at room temperature, according to conditions. Since this TiAl based alloy has such excellent impact value, if it is used for the turbine blades of an engine turbocharger or various types of turbine, it becomes possible to improve turbine efficiency due to the increase in rpm, and to contribute to lightening the weight, while maintaining durability against impact, that is, reliability.
A method for obtaining the structure 1 described above, which is one method for obtaining the TiAl based alloy of the present invention, is a production method of a TiAl based alloy characterized by comprising: a step for holding a TiAl based alloy material containing Al at least in an amount of from 43 to 48 atomic % in an equilibrium temperature range of an xcex1 phase; and a step for subjecting the TiAl based alloy material held at that temperature to high-speed plastic working, while cooling the material to a predetermined working terminal temperature.
According to such a structure, when the TiAl based alloy material is cooled from the equilibrium range of the xcex1 phase, strain, which become the starting point for the occurrence of lamellar grains, are introduced into the matrix by the high-speed plastic working. As a result, lots of lamellar grains having a small grain diameter are formed, to thereby form a fine structure.
The lower limit of the equilibrium temperature range of the xcex1 phase of the TiAl based alloy containing Al in an amount of from 43 to 48 atomic %, ranges from 1150xc2x0 C. to 1250xc2x0 C. depending on the composition. Therefore, after the TiAl based alloy is held in the equilibrium temperature range of the xcex1 phase of from 1230xc2x0 C. to 1400xc2x0 C., the TiAl based alloy is subjected to high-speed plastic working, while being cooled to 1200xc2x0 C. which is the terminal temperature of the high-speed plastic working, and distortion which becomes the starting point for the formation of lamellar grains is given to thereby obtain a fine structure. The adequate cooling rate at this time is from 50 to 700xc2x0 C./min. Moreover, a forging process, a rolling process or the like can be used as the above described high temperature plastic working.
In the production method of the above described TiAl based alloy, the TiAl based alloy material may be held at the above described holding temperature with the material being covered with a thermal insulation material, and then the TiAl based alloy may be subjected to high-speed plastic working, together with the thermal insulation material.
With such a construction, the temperature drop of the material during the high-speed plastic working can be suppressed, and normal apparatus can be applied as the apparatus for carrying out the high-speed plastic working, thereby making the process simple. Moreover, since a normal die can be used, and the size of the die can be freely set, a large TiAl based alloy product can be produced.
A method for obtaining the structure 2 described above, which is an other method for obtaining the TiAl based alloy of the present invention, is a production method of a TiAl based alloy characterized by comprising: a step for holding a TiAl based alloy material containing Al at least in an amount of from 38 to 44 atomic % in an equilibrium temperature range of a (xcex1+xcex2) phase; and a step for subjecting the TiAl based alloy material held at that temperature to high-speed plastic working, while cooling the material to a predetermined working terminal temperature.
Comparing this method with the above described method, since the material is held in the equilibrium temperature range of the (xcex1+xcex2) phase at a high temperature, and subjected to high-speed plastic working, with the soft xcex2 phase existing, workability is greatly improved. As a result, it is not necessary to cover the TiAl based alloy material with a thermal insulation material, as described above, and plastic working can be performed in a similar manner as with the normal metallic allows. Moreover, a larger structural parts can be produced.
The lower limit of the equilibrium temperature range of the (xcex1+xcex2) phase of the TiAl based alloy containing Al in an amount of from 38 to 44 atomic %, ranges from 1120xc2x0 C. to 1220xc2x0 C. depending on the composition. Therefore, after the TiAl based alloy is held in the equilibrium temperature range of the (xcex1+xcex2) phase of from 1000xc2x0 C. to 1300xc2x0 C., the TiAl based alloy is subjected to plastic working, while being cooled to 1120xc2x0 C. which is the terminal working temperature, and distortion which becomes the starting point for the formation of lamellar grains is given to thereby obtain a fine structure. The adequate cooling rate in this case is similarly from 50 to 700xc2x0 C./min. Moreover, a forging process, a rolling process or the like can be used as the above described high temperature plastic working.
The blade of the present invention is a blade using the TiAl based alloy obtained in the above described manner, having excellent ductility, and in particular, excellent impact properties.
The blade using such a material has an excellent impact value. As a result, if it is used for a turbine blade of a turbocharger or various types of turbine, it becomes possible to improve turbine efficiency due to the increase in rpm, and to contribute to lightening the weight, while maintaining reliability.
As is obvious from the above description, the TiAl based alloy of the present invention has a close arrangement of lamellar grains having a small grain diameter. Hence the metal microstructure becomes fine, to thereby improve the strength as well as toughness at room temperature, and in particular, impact properties.
Moreover, with the production method of the TiAl based alloy according to the present invention, when the TiAl based alloy material is cooled from the equilibrium temperature range of the xcex1 phase or the equilibrium temperature range of the (xcex1+xcex2) phase, distortion which becomes the starting point for the occurrence of lamellar grains is introduced into the matrix by high-speed plastic working. As a result, lamellar grains can be made fine. In addition, since the material is cooled at a relatively high speed after having been subjected to high-speed plastic working, the lamellar spacing in the lamellar structure can be made small.
The plastic workability at high temperatures is also improved, by holding the material in the (xcex1+xcex2) region. As a result, the material can be processed in normal industrial press, thereby making the TiAl based alloy material industrially advantageous.
Furthermore, if Cr or V is used as the additional element, a TiAl based alloy having excellent high temperature strength can be obtained, and if Mn is used, though the high temperature strength decreases, a TiAl based alloy having improved toughness and machinability can be obtained.
Since the blade according to the present invention has excellent impact resistance and strength, if this is used as a turbine blade for aircraft and ships, or for various industrial machines, such as gas turbines or steam turbines, it will be useful for improving the performance of the turbine and for lightening the weight.